Processing technology of airfoil blade in the hott

2022-10-16
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Research on the processing technology of wing shaped blades in five coordinate machining center

I. overview of processing methods

wing shaped blades, with wing shaped cross-section and three-dimensional distorted space, are widely used in axial-flow turbine compressors. Their processing and manufacturing have been generally completed by five coordinate CNC machines, especially by adopting high-speed data collection and storage systems and perfect data control software beds. The five coordinate machining center usually processes the blade and blade root in the way shown in Figure 1. The blade blank is clamped on the a-axis of the rotary workbench and rotates 360 degrees. It is understood that the spindle milling head swings in the c-axis direction. In the actual processing process, the pneumatic center tightens its top. The machining of blades can be divided into three steps: rough machining, semi finishing machining and finishing machining. The best way to finish the blade is by five axis linkage and high-speed spiral cutting method. This machining method has the highest efficiency and the most ideal blade shape. The blade surface part is usually processed with a face milling cutter, which has high cutting efficiency, but the face milling cutter cannot have a fixed swing angle in the c-axis direction. In order to avoid interference when processing to the blade root part, the blade near the blade root part is usually processed with a ball end milling cutter, which deflects a fixed angle in the c-axis direction to avoid the interference between the cutter and the blade root. The deflection angle in the c-axis direction is too small to avoid interference, and too large may cause interference at the blade profile on the other side. This is especially important for moving blades with large twist

II. Data preparation

for the blade profile of axial compressor and TRT axial energy recovery expansion blade in turbomachinery, the expression of the design drawing on the profile surface is usually the blade profile data of several sections, which may be spatial lattice or multi segment arc line. The data must be processed in the early stage, and the main work is smoothing, rotation and translation, so that the design coordinate system and the machine tool coordinate system can be integrated, that is, the unification of design datum and processing datum. Using high-speed spiral cutting method to machine blades requires high requirements for the smooth and continuous design of blade surface curve. The blade surface (back arc surface, inner arc surface, inlet and outlet air edge fillet) must not have sharp points, inflection points, nodes, otherwise in the state of high-speed cutting, the tool is very easy to produce large vibration in an instant, causing equipment accidents. Another case of unsmooth blade profile is that in the modeling process, although the profile of each section is a smooth and continuous function curve, when forming a three-dimensional modeling along the axis, the profile is unsmooth, with wavy fluctuations in the middle. This situation is usually corrected by adjusting the datum of each section. If the data in the same section cannot form a smooth spline curve, the original data must be modified. The specific method is to take n points on the section curve, take dense points where the curvature is large and sparse points where the curvature is small, and make the normals of these points respectively, as shown in Figure 2 and figure 3. The normal direction of each point of the smooth continuous curve in Figure 3 changes gently. Figure 2 shows the section curve formed by poor original data. The normal direction of different nodes changes violently, and the section curve is obviously not smooth. If such a section curve is used to generate three-dimensional space modeling, the blade surface is uneven, which cannot be realized in processing

III. mathematical modeling

the data of each section of the airfoil blade is expressed in a list, and the points are uniformly distributed along the sections of the airfoil circumferential direction, and the axial direction is given along the straight prime bus correspondingly. Based on the above situation, the first step of blade modeling is to carry out in a two-dimensional plane. Each section forms a closed curve in the plane, and each curve has a fixed position in the length direction of the blade. Rotate and then translate each section according to the fixed position. Generally speaking, there are two forms of blade profile. One is composed of spline curve, and there are two arc transitions at the inlet side and outlet side respectively; The second is a closed and conservative curve composed of multiple arcs. It is estimated that China will grow into the world's second largest biomedical material market curve within 10 years. The following points must be paid attention to when modeling

the section curve of the blade profile must be smooth and continuously closed. For the case that the blade profile curve is not closed, for example, the arc at the inlet and outlet edges is not tangent to the inner back arc curve, it is necessary to change the position of the arc center, or change the radius of the arc center, or make corresponding adjustments to the end points of the inner back arc curve to ensure that the chord length of the blade remains unchanged. In order to ensure that the chord length remains unchanged, you can make a straight line tangent to the chord length and the arc of the known inlet side (or outlet side), and then make two straight lines passing through the endpoint of the inner back arc curve and tangent to the inner back arc curve respectively, so as to form three straight lines and make a circle tangent to these three straight lines. This circle is tangent to the inner back arc, which plays a smooth transition role, and also ensures that the chord length remains unchanged

the edges of inlet and outlet air edges of blades should be two smooth curves respectively. Generally, for a blade with completely correct blade profile data, after modeling, it should be as shown in Figure 4, and the edges of inlet and outlet air edges are two smooth curves. However, sometimes the edges of the inlet and outlet edges are wavy. The method to solve this problem is to select more than 5 endpoints on the inlet and outlet edge of the cross-section to form a quadratic curve, which is second-order continuous, so as to correct the inlet and outlet endpoint data of other cross-section data

calculation of tool overcut there are two ways to avoid tool overcut, that is, changing the tool diameter or changing the cutting angle. The blade surface with large curvature is prone to over cutting. For convex surface machining, the over cutting phenomenon is not easy to occur when the tool cluster cuts along the normal vector of the surface; For concave surfaces, cutting along the normal vector of the profile with the tool cluster will produce overcut under the influence of the radius of curvature. At this time, the method of changing the tool radius should be preferred to avoid overcut. Calculating the tool diameter and cutting angle while modeling can greatly improve the programming efficiency. As shown in Figure 5, the method is to evenly take n points on the shaped closed blade section curve, and then define an imaginary tool and an imaginary cutting angle on the first point, so that the tool passes through each point on the section according to the determined cutting angle in a recursive way, and observe whether there is overcut. If so, modify the tool diameter and cutting angle. Since the cutting condition observed at this time is in the two-dimensional space, only for a certain section, and cannot reflect the actual three-dimensional machining condition, further technical treatment is needed, that is, the sections of two adjacent blades are projected in the same plane. If the section distance is greater than the tool diameter, the tool and the sections of two adjacent blades will not have overcut on the projection drawing, Then it can be considered that the hypothetical tool diameter and cutting angle are appropriate. In order to improve the cutting efficiency, large diameter tools should be used as much as possible without overcutting

establishment of coordinate system a three-dimensional coordinate system must be established for any part to be processed on a CNC machine tool. In actual machining, reasonable establishment of coordinate system can simplify programming and facilitate tool setting. Generally, it is necessary to ensure that the design datum is unified with the machining datum. As far as possible, the X coordinate system is established on the blade axis on the machining center, that is, the X axis coincides with the blade axis, which is equivalent to determining the origin of the Y axis and the Z axis. For rotor moving blades, there is a smooth connection between the blade profile and the blade root, which is called the transition arc. The part of the transition arc located at the blade root is usually a cylindrical surface or a spherical surface, and the origin of the X axis can be determined on the spherical center of the above cylinder or ball. For the stationary blade of the rotor, the part where the transition arc is located at the blade root may be cylindrical, spherical or inclined. If it is a cylindrical surface or a spherical surface, the determination method of the origin of the x-axis is the same as that of the moving blade; If it is an inclined plane, the determination method of x-axis origin can be determined according to the tool setting

extension and interception of blade profile in general, in the design drawing of airfoil blade, only the tabular curve data of several sections are given, and the actual blade profile may be longer or shorter than the one determined by the given section. In the first case, the blade profile should be extended; in the second case, the blade profile should be intercepted. Relatively speaking, it is better to deal with the interception of the blade shape. Only a plane or composite surface is needed to intercept the blade shape at a specific position to obtain a new section, and the required blade shape entity can be formed by using the data of the new section. When extending the blade profile, it is also necessary to smooth the blade profile once. The fairing of the above method is only a plane curve, and after the extension of the blade profile, it is a space curve, that is, fairing its projection curves in two or three coordinate planes respectively. In fact, generally, it is only necessary to project the spatial curve onto two planes, and then synthesize the spatial curve after smoothing the two plane curves respectively (that is, three-dimensional is treated as two-dimensional). Practice has proved that in general, the projection curve of a spatial curve in each coordinate plane is smooth, and the spatial curve is also smooth

when machining the blade surface, the motion of three linear axes and two rotating axes need to be synthesized to realize the motion trajectory of the required contour. In the actual calculation process, the three parameters shown in Figure 6 can be appropriately adjusted to meet the technical conditions of the blade. MND is used to determine the angle to control the blade profile error. The blade profile curve of each section can be divided into countless segments, and the curvature of each segment can be considered to be the same. The value of MND directly determines the density of adjacent two points during interpolation. The smaller the value of MND, the denser the adjacent two points, and the higher the accuracy of the processed blade profile. MCD controls the linear distance between two adjacent points, and errcdr controls the chord height difference between two adjacent points. As with MND, different MCD and errcdr values determine different density. In the cutting parameters, because the spatial surface is generally processed by row cutting method, the row spacing and step length must be calculated or determined

for example, the line spacing s of 1500MPa is directly related to the residual groove height on the machined surface. If it is large, the surface roughness is large, but if s is too small, although it can improve the machining accuracy and reduce the difficulty of clamping repair, the program is lengthy, the machining time is doubled, and the efficiency is reduced. Therefore, the choice of line spacing s should strive to be just right

cutting angle when machining a blade profile with a face milling cutter, the selection of the included angle between the bottom surface of the face milling cutter and the tangent direction of the cutting point of the blade surface is very important. If it is inappropriate, it is very easy to produce overcut. The drawing method is usually used to determine the cutting angle in actual production. The specific method is to use the drawing method to make the profile of a certain section of the blade as shown in Figure 5, and then take n points evenly on the section, taking one of them as the imaginary cutting point. At the same time, according to experience, determine an arbitrary cutting angle, and make the cross-section of the tool, and then use the circular statement to make the tool pass through n points, and observe whether there is overcut. If so, adjust the cutting angle, and repeat the above work, Until there is no overcut

the specific spindle speed, feed rate and cutting depth to be used depends on the blade material, tool diameter, processing mode and other conditions. Five axis blade machining center usually adopts high-speed cutting

v. tool path simulation

the NC program generated by post-processing must be automatically simulated by relevant software

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